Fix GridFSURIStore and increase testing

[gpetretto]

While testing GridFSURIStore I discovered that there are several potential sources of errors due to the fact that it uses methods of the baseclass (GridFSStore), even if some attributes are not defined. Also there are no safeguard for accessing the _files_store attribute in case the store is not connected.

This PR addresses these issues and modifies the gridfs tests so that all the tests are executed on both the GridFSStore and GridFSURIStore. This should hopefully reduce the chances of regression.

Also I noted that the ssh_tunnel is not taken into account in either of the "URI" stores. I kept the argument in the __init__ to maintain the same API as the base classes, but it explicitly raises if passed.

[codecov]

Codecov Report

Attention: Patch coverage is 79.16667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 88.84%. Comparing base (2632269) to head (7415557).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/maggma/stores/gridfs.py	80.95%	4 Missing ⚠️
src/maggma/stores/mongolike.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1037      +/-   ##
==========================================
- Coverage   88.91%   88.84%   -0.07%     
==========================================
  Files          46       46              
  Lines        3997     4017      +20     
==========================================
+ Hits         3554     3569      +15     
- Misses        443      448       +5     

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[rkingsbury]

Thanks @gpetretto